Sea-level records from the U.S. mid-Atlantic constrain Laurentide Ice Sheet extent during Marine Isotope Stage 3

ARTICLE Received 7 Dec 2016 | Accepted 11 Apr 2017 | Published 30 May 2017 DOI: 10.1038/ncomms15612 OPEN Sea-level records from the U.S. mid-Atlantic constrain Laurentide Ice Sheet extent during Marine Isotope Stage 3 T. Pico1, J.R. Creveling2 & J.X. Mitrovica1 The U.S. mid-Atlantic sea-level record is sensitive to the history of the Laurentide Ice Sheet as the coastline lies along the ice sheet’s peripheral bulge. However, paleo sea-level markers on the present-day shoreline of Virginia and North Carolina dated to Marine Isotope Stage (MIS) 3, from 50 to 35 ka, are surprisingly high for this glacial interval, and remain unexplained by previous models of ice age adjustment or other local (for example, tectonic) effects. Here, we reconcile this sea-level record using a revised model of glacial isostatic adjustment characterized by a peak global mean sea level during MIS 3 of approximately  40 m, and far less ice volume within the eastern sector of the Laurentide Ice Sheet than traditional reconstructions for this interval. We conclude that the Laurentide Ice Sheet experienced a phase of very rapid growth in the 15 kyr leading into the Last Glacial Maximum, thus highlighting the potential of mid-field sea-level records to constrain areal extent of ice cover during glacial intervals with sparse geological observables. 1 Harvard University, Department of Earth and Planetary Sciences, Cambridge, Massachusetts 02138, USA. 2 Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, Oregon 97331, USA. Correspondence and requests for materials should be addressed to T.P. (email: ). NATURE COMMUNICATIONS | 8:15612 | DOI: 10.1038/ncomms15612 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15612 R econstructing the pace of ice growth towards the Last Glacial Maximum (LGM, 26 ka) is critical to our understanding of ice age climate and ice sheet stability. Nevertheless, global ice volume, or equivalent global mean sea level (GMSL), and the corresponding geographical distribution of ice remain uncertain through Marine Isotope Stage 3 (MIS 3; 60–26 ka) leading into the LGM1,2. Oxygen isotope records from marine sediment cores provide a proxy for global ice volume after correcting for temperature-dependent fractionation3, however uncertainties in this correction and other complications in mapping isotope values to ice volumes have yielded estimates of peak MIS 3 GMSL that range from  30 to  60 m relative to present day1. Geological records of sea level during MIS 3 are sparse because ancient markers in the far field of former ice sheets are presently submerged, while those in the near field have been erased by the subsequent advance and retreat of the major continental ice sheets4,5. Moreover, glacial isostatic adjustment (GIA) and tectonic uplift contaminate the present-day elevation of available sea-level records6,7. Studies that applied GIA modelling to fit oxygen isotope records and geological sea-level markers have published discordant inferences of peak MIS 3 GMSL, varying from  85 m (ref. 8) to  55 m (ref. 9), and most recently  37.5±7 m (ref. 10). The geological markers of Pleistocene sea-level oscillations extending from Virginia to North Carolina in the Albemarle Embayment (Fig. 1), on the Laurentide Ice Sheet’s (LIS) peripheral bulge, require a re-evaluation of ice volume and extent during MIS 3. This record indicates that MIS 3 relative sea level (RSL) reached present-day levels from B50 to 35 ka in this region11–16 (Fig. 1; Supplementary Table 1), but GIA calculations predict that these markers should presently be found as much as B70 m below sea level8. Tectonic uplift of the markers is insufficient to explain their present-day elevation17,18 and sediment compaction has led to only minor subsidence in this region19. MIS 5e MIS 5c Here, we present a new set of GIA calculations that explore the sensitivity of the predictions to peak GMSL and LIS geometry during MIS 3. We conclude that a revised GIA model can reconcile the MIS 3 sea-level record at the Albemarle Embayment under two conditions: (1) peak GMSL reached near  40 m and (2) the eastern sector of LIS was significantly reduced during MIS 3 compared with previous reconstructions of ice extent. Results The U.S. Mid-Atlantic sea-level record. The Albemarle Embayment geological record includes interfluvial, estuarine, intertidal and shallow marine lithofacies arranged in depositional sequences that record repeated sea-level highstands dated primarily by optically stimulated luminescence to MIS 5e, 5c, 5a and 3 (ref. 11) (Fig. 1; Supplementary Table 1; Supplementary Note 1). We adopt the minimum elevation of terrestrial facies and the maximum elevation of marine facies as upper and lower bounds, respectively, of MIS 5a (B80 ka) and mid-MIS 3 (50–35 ka) sea level. For the MIS 5a data (Fig. 1; Supplementary Table 1), we bound a cluster of sea-level data from 2.5 to 7 m in agreement with previous assessments of sea-level records in the region20. We assume that rare terrestrial markers found at elevations below this range do not represent a constraint on the MIS 5a highstand, but rather a lower sea level reached during late MIS 5a or MIS 4. Furthermore, calculations described below (and detailed in Supplementary Note 3) demonstrate that RSL predictions for MIS 3 are relatively insensitive to the height of sea level during MIS 5a. For the MIS 3 interval spanning 50–35 ka, three marine indicators constrain RSL to be above  0.9,  3 and  2 m (ref. 11). We thus adopt the elevation of the highest of these MIS 5a MIS 3 Terrestrial (upper bound) 15 Tidal Marine (lower bound) 10 11 5 –5 –10 39°N 110 km 2.5 38°N Elevation (m) 0 Best 11 Scott 11 Parham et al. 12 Parham et al. 13 Mallinson et al. 11 Parham et al. 11 Parham et al. 14 Culver et al. 15 Cronin et al. 16 Mixon et al. 2.0 1.5 –15 Elevation (km) 1.0 37°N 36°N 0.5 0 –20 –0.5 –1.0 –1.5 35°N –25 –2.0 34°N 80°W 120 –2.5 79°W 110 78°W 77°W 100 76°W 90 75°W 74°W 70 80 Time (ka) 60 50 40 30 –30 Figure 1 | Present elevation of sea-level indicators from the Last Interglacial to the Last Glacial Maximum for the Albemarle Embayment. Field localities are shown by yellow dots on the inset map. Upwards pointing triangles represent marine indicators (lower bound), downwards-oriented triangles represent terrestrial indicators (upper bound), and circles designate tidal facies. Error bars span 2-s age uncertainties on individual sea-level data. Marine Isotope Stages 5e, 5c, 5a and 3 are labelled at 120, 100, 80 and 60–26 ka, respectively. The shaded region covers the time interval examined within the present analysis and the orange rectangles mark the bounds on MIS 5a and MIS 3 sea level based on the plotted data (MIS 5a: 2.5–7.5 m; MIS 3:  1 to 1 m). The white star on the inset map marks the location of RSL predictions presented herein. 2 NATURE COMMUNICATIONS (...truncated)